Forces and Motion

Getting out of shape

Changing speed

Going round in circles

What do forces change?

• They change an object’s– speed– shape– Direction (link)

Forces everywhere

• A force is a push or a pull exerted on an object• So many sorts ...

– Pull of Gravity– Push of ground upwards (Reaction)– Electrostatic– Magnetic– Upthrust (on floating objetcs)– Friction/air resistance/drag– any more?

More than one force• Do things always move?• So what’s happening?• Forces can add together or oppose and cancel• What else might happen?

Balanced forces

– Cancel each other– Have a zero resultant force

• What won’t happen?• What else might happen?• Balanced forces don’t affect overall motion

• As soon as something starts moving one force in particular appears out of nowhere and makes life awkward

• Friction: but is it always a nuisance?– Rubber needs to grip the road or

wheels spin and you get nowhere

– Brakes need to work so you can stop

• Understanding leads to control!– Curving a football– F1 cars

Sometimes we need unbalanced forces

Unbalanced forcesForce A Force B Force diagram Size and direction of the

resultant force

1 12 N to the left 7 N to the right

2 7N to the right 12 N to the right

3 7N to the left 7 N to the right

4 12 N to the left 12 N to the left

What about Forces acting on objects already moving?

• Balanced forces?

• Stays still

• Unbalanced forces?

• Starts moving

• Already moving and unbalanced forces?

• Speed carries on changing.

• Already moving and balanced forces? Imagine removing friction ...

How do things move?

• In a straight line, at rest or constant speed, unless– an unbalanced force acts

• A resultant (balanced) force causes a change in speed or a change in direction or both.

Getting Out of Shape

Forces

How do things stretch?

• Once twisted, plasticine stays in the new shape or breaks - PLASTIC

• A rubber band regains its shape after being stretched and released - ELASTIC

• An elastic material when released after being deformed returns to its original shape and size.

• Glass snaps - BRITTLE

Investigating How Springs Stretch

• Set the apparatus up as shown in

the diagram

• Place a 100g mass hanger on the

end of the spring.

• Line up the top of the ruler,

reading 0 cm, with the bottom of

the mass hanger

• Now wear goggles!

• Add masses, 100g at a time and

record the new length for each

load, (so get a table ready!).

• Stop at a total of 1 kg

Label s needed!

Good springs stretch evenly

• Provided the elastic limit is not reached:

– Straight line graph

– Passing through the origin

This is described as:

• a proportional relationship

• Doubling the load force doubles the extension.

Using Springs

• The way they stretch is so

regular, they can be used to

make a newtonmeter to

measure forces.

• Mark the zero position

• Add 1 kg and mark the new

position

• Divide the gap into 10

divisions

• Weigh something!

• This is called calibration

Stretching Rubber

• Try stretching a rubber band gently then strongly.

• Try sketching the sort of graph you think you could draw for force against extension.

• Then hold it against your forehead and stretch it suddenly. What do you feel?

Resultant forces

• Apparatus and materials

• For each student group:  • Retort stand, boss and clamp • Mass hanger plus masses (100 g) • Metre rule • Selection of rubber bands, elastic cord • Marker pen

Plan• Measure the distance between the marks when the

band is not stretched

• Add 50 or 100g and measure it again.

• Go on adding loads and recording the distance up to 600g

• Plot a graph of Load against Length

How do I change speed?

Speed• Speed is a measure of how quickly

something is moving

• Actually, the above formula really tells you the average speed during the time interval

• As the time interval gets smaller, you get closer to calculating the instantaneous speed.

takentime travelleddistance

speed

• This journey can be broken up into 5 stages.

• How far does he go in each stage?

• What is the total distance travelled?

• What is his average speed?

• Start• Subjects• Physics• Force and Motion• Displacement – time graphs

• In the “Forces and Motion” program sketch out this curve, as near as you can, and watch how the car moves.

• Design a journey for yourself. Be patient and learn how the program works.

• Note how far the car has gone at the end.

• Design a journey that gets you back to the starting point, but is different in how it gets you “there” and “back”.

• Print these off if you can!

Representing motion• We can do this with a travel graph

– a plot of distance travelled vs time• We must define a starting position and

direction– At starting position the distance=0– One direction of travel is positive, the opposite

direction is negative

Speed and Velocity

• The velocity of an object gives its instantaneous speed and direction– (This is called a VECTOR)– Speed on its own is a scalar.

• As with displacement, the sign of the velocity indicates the direction– Without direction, speed is a scalar– a negative velocity means speed in the

opposite direction

Distance-time graphs

Note: distance can also become negative, if object travels in the opposite direction

How would you represent something getting slower?

t

x

Speed and Velocity

• Going from A to B: + velocity

• Going from C to F: - velocity

Ben Johnson: 1987 world record run

• Copy the table

• Draw a distance-time graph of the 100 m run.

• What is his average speed?

• What speed does he reach in the fastest 10m?

• What is his speed in the slowest 10 m?

• Describe from the graph how his speed changes during the race.

Time taken / s Distance run / m

0 0

1.65 10

2.76 20

3.71 30

4.63 40

5.52 50

6.38 60

7.23 70

8.09 80

8.96 90

9.83 100

0 0

1.65 10 6.060606

2.76 20 9.009009

3.71 30 10.52632

4.63 40 10.86957

5.52 50 11.23596

6.38 60 11.62791

7.23 70 11.76471

8.09 80 11.62791

8.96 90 11.49425

9.83 100 11.49425

0

20

40

60

80

100

120

0 2 4 6 8 10 12

Series1

Calculating speed• The slope of the graph gives the speed

(strictly the velocity)–

• The steeper the line, the higher the speed

takentime travelleddistance

slope

Slope = 60/10 = 6.0 m/s

(a)

(b)

(c)

(a)

(c)

(d)

Slope = -100/25 = -4.0 m/s

Slope = 40/15 = 2.7 m/s

(d)

Slope = 0/5 = 0.0 m/s(b)

Distance-time graphs• Try to describe the motion shown in the graph

– What does the slope of the line represent?– What does the slope of the dotted line tell you?

Distance-time graphsConstant speed forward

stationary

Constant speed backwards

After 160 minutes, we are back where we started

Slope=average speed of return journey

Slope = speed

Speed=5/0.42=11.9 km/h

• Add up all the distances in each stage to get a total distance covered in the journey.

• What was his average speed over the whole journey?• Can you work out his fastest speed forwards?• Can you work out his fastest speed coming back?

Earth’s Gravity Field• The weight of something

is a measure of how strongly the Earth’s gravity pulls it down.

• The region where gravity acts is called a gravitational field.

• Gravity pulls more strongly on more massive objects, so we record the weight per kg as the field strength.

How does gravity work?

• Isaac Newton realised the significance of two simple ideas:– It is the same force that

holds moons in orbit as pulls things down on earth

– Every piece of matter attracts every other piece of matter

Size – or mass – decides how strongly gravity pulls

g• “g” represents the

strength of gravity, it varies from place to place.

• However far away you go, it just gets weaker and weaker but never vanishes completely.

Earth: 10 N/kg

Moon: 1.6 N/kg

Mars: 3.8 N/kg

Venus: 9.1 N/kg

Mercury: 3.8 N/kg

Pluto: 0.6 N/kg

Sun: 271 N/kg

100km asteroid: 0.04 N/kg

How come moons stay in orbit?• Imagine a giant

cannon firing a cannonball faster and faster

• The pull of gravity makes sure it lands ...

• Until it is going fast enough to fall round the earth!

• Gravity keeps the ball in orbit

Comet orbits• If we fire a satellite into orbit and don’t get

the speed just right, it starts to overshoot before being pulled back...

• This distorts the orbit into an ellipse.• Comets move in clear

ellipses• Most planetary orbits

are almost circular.

How to satellites move round an ellipse?

• As they climb higher they lose KE and slow down.

• As they fall lower they lose GPE and gain KE, speeding up

• Wherever they are gravity pulls them towards the centre of the Earth

Gravity holds the Universe

together• Moons go round

planets• Planets go round stars• Stars revolve round

galactic centres• Hundreds of billions of

stars in a galaxy• Hundreds of billions of

galaxies

Calculating speeds in orbit

• How fast is the Earth moving?

• Radius of orbit: 150,000,000 Km

• Time of orbit: ?• Speed is:

distance travelled

time taken

• 2πr / T• 2x3.14x150,000,00

365x24• km/hr

Research for yourself